When a steering wheel of an automobile is operated, a steering shaft is rotated, and rotation of this steering shaft is converted to a reciprocating motion and changes the directions of wheels. An electric power steering device for assisting such a steering operation by using driving force of a motor is disclosed in, for example, JP-A 2001-206230.
In such an electric power steering device, as shown in FIG. 7, a reduction device for transmitting the rotary driving force of a motor has a motor 5A, a rotational shaft 7B connected to an output shaft 7A of the motor 5A via a coupling sleeve 8A, a worm gear 61 fixed on the rotational shaft 7B, and a worm wheel gear 62 engaged with the worm gear 61. The worm wheel gear 62 is fixed on a steering shaft 3 or a shaft connected to the steering shaft 3. Here, the output shaft 7A is supported by two bearings (not shown) in the motor 5A, and the rotational shaft 7B is rotatably supported by a first bearing 81A and a second bearing 82A on either end of the worm gear 61.
Therefore, when a steering wheel is operated by a driver, if the operation is detected by a torque sensor (not shown), the motor 5A is started, and the rotary driving force of the motor 5A is reduced and transmitted to the steering wheel 3 via the output shaft 7A, the rotational shaft 7B, the worm gear 61, and the worm wheel 62 so as to assist the steering operation.
However, in the conventional electric power steering device, the rotational shaft 7B is connected to the output shaft 7A of the motor 5A, which has been assembled as a completed product, via the coupling sleeve 8A. Hence, it is necessary to support the output shaft 7A by using the two bearings in the motor 5A and to support the rotational shaft 7B by using the two bearings 81A and 82A, so that no less than four bearings are necessary in total. Thus, regarding the conventional electric power steering device, the number of components is large, the cost is high, and the device is larger and heavier. Additionally, since the device has a large number of bearings, the rotation of the steering shaft 3 requires a strong force, resulting in poor steering feel. Furthermore, in the conventional electric power steering device, the output shaft 7A of the motor 5A and the rotational shaft 7B of the worm gear 61 are connected by spline grooves via the coupling sleeve 8A. As spline machining is necessary, the cost is increased. Additionally, when a rattle occurs on a connected part of the spline grooves, an unusual noise appears when the motor 5A is rotated, and the response of the worm gear 61 to the rotation of the motor 5A is degraded.
In view of the above problems, the present invention has as its object the provision of a power transmission device, an electric power steering device, and a method of manufacturing the devices whereby a lower cost, a smaller size, a reduced weight, and improvement in response at the transmission of power are achieved by reducing the number of bearings, and unusual noise can be also prevented.